Method of reducing magnesium oxide



e a Patented on. so, 1945 a UNITED STATES PATENT OFFICE- I METHOD OFREDUCING MAGNESIUHOXIDE Hugh 8. Cooper, Cleveland, Ohio, assignor, bydirest and mesne assignments, to Acme Aluminum Alloys, Inc., Dayton,Ohio, a corporation No Drawing. Application July 31, 1942,

Serial No; 453,051

7 Claims. (01. 75-67) This invention relates to metallurgy and moreparticularly to thermal reduction processes'and has for its object theprovision of an improved thermal reduction process for the productionvoi metallic magnesium. v

Another object is the provision oi a relatively low temperature thermalreduction process for producing metallic magnesium.

form the intermetallic compound, calcium silicide Still another objectis to provide a low temperature vacuum distillation method for theduction of metallic magnesium. I Other bbjects will be apparent as theinvention pro- .is more fully hereinafter disclosed.

In accordance with these objects I have discovered that calcium silicideis an efiective thermal reducing agent for magnesium oxide attemperatures within the'range 900-1l50 C. and at reduced pressuresapproximating millimeter of mercury. v I

Heretoiore in the art, both calcium and silicon have been employed asthermal reducing agents for metal oxides at atmospheric and reducedpressures, and both reducing agents have been employed as thermalreducing agents for magnesium oxide. Each when used alone have certaindefects which do not appear when the two agents have been reactedtogether to form the silicide intermetallic compound.

For example, silicon whenused alone as a thermal reducing agent requiresa reaction temperature in the neighborhood of 1400-1500 C. to

reduce magnesium oxide at which temperature the silicon dioxideresulting, being strongly acidic reacts readily with unreduced magnesiumoxide forming a thermally stable magnesium silicate. This reactiongreatly lowers the eillciency oi the reducing reaction and interfereswith the yield of magnesium obtainable by the process. The temperaturepithe reaction is in excess of tempera-'- tur'es easily obtained andrequires the use of special high temperature equipment which at presentis diillcultly obtainable.

On the other hand when calcium is employed as a reducing agent formagnesium oxide, while the temperature oi reduction is materially lower,

the diiiiculty in fragmenting the calcium into small sized particlesrequired tor the reduction and the high chemical activity of the calciumwhen reduced to such small particle sizes with the atmospheric gases andwater vapor, coupled with the high vapor pressure of the calcium at thethermally reducing temperature of magnesium oxide and the high cost ofthis reducing agent,

renders the use 01 this method economically'unpractical.

When calcium and silicon are first reacted to (CaSiz) all of theadvantages of the low temperature reduction of magnesium oxide bycalcium is retained with none of the attendant disadvantages, and therelatively low cost of the calcium silicide as compared to the cost ofeither calcium orsilicon render the process decidedly commerciallypractical.

In the reductionof magnesium oxide by calvessel, or retort and briquetcharge therein is heated to a temperature within the range 900- 1150 0.,preferably about ll00 0., and is maintained at this temperature for anextended time interval adapted to obtain the maximum yield of magnesiumat minimum cost.

In the above process, many factors enter into the eiilciency oi theprocess. The most important factor is the relative proportions of magneaslum oxide (MgO) and calcium silicide (cash) in the mixture. The secondmost important iactor is the temperature of reduction. A third importantfactor is the length of the path of travel for the magnesium from thepoint of reduction and vaporization to the point of condensation. Otherfactors are the mode oi heating the charge and the retort apparatusemployed.

With respect to the relative proportions of magnesium oxideto calciumsilicide, the primary reduction reaction under the low pressureconditions invloved is between the calcium content of the silicide andthe magnesium oxide.

Under the low pressure conditions present, the calcium content of thecalcium silicide tends to be vaporized and the calcium vapors at thetemperature of heating react with the magnesium oxide resulting in theformation of calcium oxide and magnesium metal vapors which vapors aredistilled off from the reaction mixture.

However, the silicon when set free by the calcium, vaporization and itsreaction with magnesium oxide, is in a highly reactive condition, and isreactive as a reducing agent towards both calcium and magnesium oxide.This circumstance results in the reduction of some of the magnesiumoxide by the silicon but the predominant reaction is a reduction of thecalcium oxide by the silicon for re-use in the reduction of moremagnesium oxide.

The extent to which this silicon regeneration oi calcium vapors may govaries widely depending upon the fineness of subdivision of themagnesium oxide and the calcium silicide, upon the intimacy of theadmixture of the two reacting constituents and upon the pressureemployed in compacting the mixture together. With any given particlesize of mixed oxide and silicide and intimacy of mixture there is anoptimum I pressure to provide the right porosity for vacuum Mgo-l-CasizMg-l-Cao-i-siz 40.36 96.8-

Such a proportioning is roughly 2 parts calcium silicide to each partmagnesium oxide. The calcium silicide content may be reduced to as lowas 2 parts silicide to each part magnesium oxide without markedlyeffecting the efliciency the interior I of the reduction reaction orextending the time interval of reduction inordinately. By the use of anexcess of calcium silicide over the 2 to 1 ratio above indicated therate of reduction is accelerated and the time interval for completereduction of the magnesium oxide is reduced.

Under any given conditions it is necessary to determine the preciseratio of reacting constituents that produces the best results in themost economically practical manner.

In the thermal reduction of magnesium oxide in accordance with thepresent invention, 1 have found that the most economically practicalapparatus to be employed is that which is described and claimed in myco-pending application Serial No. 453,984, filed August 7, 1942, whichapplication is assigned to the same party and in the same manner as isthe present application.

In the apparatus ofthis said co-pending application the charge ofbriquets is disposed in a graphite-clay crucible which is locatedcentrally within a. vertically sustained tubular metal chamber which isheated externally and uniformly to I the desired reaction temperature.

The upper and lower ends of the tubular chamber extend beyond the heatedcentral portion enclosing the crucible and are provided with removableend closure members provided with vacu- I um seal joints securing thesame in closure posievacuate the interior of the chamber to the de siredlow pressure. The location of the condensing surface in relativelyclosed spaced relation to the open end of the crucible provides a meansfor maintaining the condensed magnesium at a temperature approximatingbut below the solidification temperature or melting point whereby graingrowth is promoted with the formation of large sized crystals andaggregates which are resistant to oxidation except superficially onexposure to the air during the transfer of the condensed metal from thereducing chamber to a bath of molten salt for fusion and casting intoingot form.

The exterior surface of the tubular metal chamber is protected fromoxidation at the temperature of heating by means of an enclosingrefractory sleeve disposed in close space relation to the exteriorsurface of the chamber, and by providing in the said space gap aprotective or non-oxidizing atmosphere such as H2, N2. cracked ammoniagases, illuminating gas, coke oven gas, or the like.

For more specific description of the preferred apparatus for thepractice of the present invention, reference should be made to thisdescription of said co-pending application.

It is believed apparent from the above dis-.

closure that the method of thermally'reducing magnesium oxide by thereducing agent calcium silicide at temperatures within the range 900-1150 C. and at reduced pressures of the order of less than onemillimeter of mercury, may be varied widely without essential departuretherefrom, and all such modifications and adaptations thereof arecontemplated as may fall within the scope of the following claims:

What I claim is:

1. In the reduction of magnesium oxide with a metallic reducing agent atelevated temperatures and under reduced pressures, the improvement whichcomprises forming porous aggregates consisting of an intimate mixture ofmagnesium oxide and calcium silicide, each in finely divided state andin the relative proportions of one part oxide to about 2 parts silicideand heating the said aggregates to a temperature within the range900-1150 C. in a container closed tothe atmosphere and evacuated to apressure of less than one millimeter of mercury.

2. The method of producing metallic magnesium which comprises forming anintimate mixture of magnesium oxide and calcium silicide, each in finelydivided condition, and in the relative proportions of one'part oxide tofrom 2 to 3 parts silicide, compacting the said mixture to porousaggregates, heating the porous aggregates to a temperature within therange 900-1150 C. in a'container closed to the atmosphere and evacuatedto a pressurebelow about one millimeter of mercury and collecting themagnesium vapors within said container upon a condenser surfacemaintained at a temperature silicide with finely divided magnesium oxidein the relative proportion of 2 to 3 parts silicide to each one partoxide, compacting the said mixture into porous aggregates, heating thesaid aggregates to a temperature within the range 900- denser disposedinrelatively close spaced relationship to the said aggregates, the saidsurface being maintained at a temperature approximating but below themelting point 01' magnesium.

4. The method of producing magnesium which comprises forming a mixtureor substantially pure magnesium oxide and calcium silicide each infinely divided condition. the relative proportions 01' oxide to silicidebeing within the range 1 to 2 and 1 to 3, compacting the mixture toporous aggregates and heating the said aggregates to a temperaturewithin the range 1150 C. in a container closed to the atmosphere andevacuated to a relatively low pressure, thereby to effect vaporizationof the calcium from silicide combination and the reduction of themagnesium oxide by the vaporized calcium and the reduction of thecalcium oxide product by the silicon residue of the said silicide.

5. The method of claim 4. wherein the magnesium vapor evolved from saidheated aggregates is collected upon a condenser surface mainthe meltingpoint of magnesium.

tained at a temperature approximating but below 6. The method oi claim4, wherein the magnesium vapor evolved from said heated aggro gates iscollected upon a condenser surface sustained in relatively close spacedrelationship to the said ag regates and maintained at a temperatureapproximating but below the melting nesium vapors evolved from the saidCheated aggregates upon a condenser surface maintained at a temperatureapproximating but below the melting point 01' magnesium disposed in aposition providing a relatively short path oi travel for said vapors.

HUGH 8. COOPER.

